Hitherto, a leakage of flammable gas has been detected by bringing sucked gas to direct contact with a sensor portion, and by measuring a gas concentration based on a change in value of electrical resistance, current, etc. However, the known gas detector is of the sensor type that a region capable of being monitored by one detector is narrow and a gas leakage cannot be detected unless the leaked gas reaches the detector. Accordingly, there has been a risk that, in spite of the event of a gas leakage, an alarm is not issued depending on the direction of wind and the position where the detector is installed. Another problem is that, in a gas refinery or the like, a very large number of gas detectors must be installed and a substantial cost is required (see Patent Reference 1).
On the other hand, to overcome the above-mentioned problem, there is proposed a gas visualizing device for monitoring the presence of a gas leakage from a far distance. The proposed gas visualizing device comprises a laser beam source for irradiating an infrared laser beam that contains an absorption wavelength of gas to be measured, and an image sensor for imaging absorption of the infrared ray reflected from the background by leaked gas so that the leaked gas is displayed in the form of a two-dimensional visible image.
However, such a known gas visualizing device requires a very large-sized and high-power laser beam source and therefore has a serious problem in point of cost. Another problem is that the displayed two-dimensional image is greatly affected by weather conditions and temperatures, and a difficulty arises in discriminating the occurrence of a gas leakage from shinning of sunlight. For those reasons, the known gas visualizing device has not been suitable for monitoring a gas leakage in practical fields (see Patent Reference 2).
Patent Document 1: Japanese Patent Laid-open No. 6-307967
Patent Document 2: Japanese Patent Laid-open No. 6-288858